The present invention relates to a piezoelectric element. More particularly, the present invention relates to a piezoelectric portion which has very high piezoelectric properties, and which has a high rate of increase of displacement as an actuator in the case of applying a large electric field and a high decomposability as a sensor in the case that a great force is applied.
In recent years, piezoelectric elements have been used in ink jet printer heads, speakers, microphones, etc.
As the piezoelectric elements, there are generally known those comprising a ceramic substrate, a piezoelectric portion made of a piezoelectric ceramic composition, formed on the substrate, and electrodes electrically connected to the piezoelectric portion. As to the piezoelectric ceramic composition constituting the piezoelectric portion, various improved compositions have been disclosed.
For example, a Pb(Mg1/3Nb2/3)O3xe2x80x94PbTiO3xe2x80x94PbZrO3 ternary system solid solution composition and a piezoelectric ceramic composition wherein part of Pb in the above composition is replaced by Sr, La, etc., were disclosed [Japanese Patent Publication S44-17103, Japanese Patent Publication S45-8145]. These compositions are improved in piezoelectric properties (e.g. piezoelectric d constant) which are the most important factor determining the piezoelectric element properties, and a piezoelectric element having superior piezoelectric properties is expectable.
When a piezoelectric element is produced actually by superposing a piezoelectric material made of the above piezoelectric ceramic composition on a ceramic substrate and then subjecting the piezoelectric material to heat treatment, the obtained piezoelectric portion has a low density, and therefore, there have been pointed out such a problem that the piezoelectric element has a low flexural displacement or, when a voltage is applied thereto, it causes dielectric breakdown at the low density area. In particular, this problem is remarkable in the case of a piezoelectric element having a layered structure that a negative electrode and a positive electrode are interposed between piezoelectric portions alternatively, and its improvement has been strongly demanded.
In addition, sufficient piezoelectric properties cannot always be obtained with a piezoelectric portion made of the aforementioned piezoelectric ceramic composition, and there have been such a problem that when a voltage is raised to increase a flexural displacement, an increase of the flexural displacement obtained is very small relative to an increase of voltage on a side of a high electric field of 4 kV/mm or more.
Further, durability of the piezoelectric element is not sufficient when it is used for a long time under such a condition that a large flexural displacement is caused, thereby causing breakage of a piezoelectric portion and exfoliation between a piezoelectric portion and the substrate or an electrode.
Hence, there has been proposed a piezoelectric element produced by previously heat-treating a piezoelectric material made of the above piezoelectric ceramic composition to produce a piezoelectric portion, and then attaching this piezoelectric portion onto a ceramic substrate (Japanese Patent Application H11-29357).
In this piezoelectric element, attention was paid to a fact that use of a ceramic substrate impairs the densification of a piezoelectric portion, and this piezoelectric element was accordingly improved in piezoelectric properties by means of obtaining a dense piezoelectric portion where a piezoelectric material made of a piezoelectric ceramic composition was previously heat-treated.
In this piezoelectric element, however, it is necessary to use an inorganic or organic adhesive at the time of attaching the piezoelectric onto the ceramic substrate; therefore, there have been such problems that the adhesive impairs the vibration transmittability between the ceramic substrate and the piezoelectric or the adhesive components infiltrate into the piezoelectric or the ceramic substrate, deteriorating their properties.
In addition, in this piezoelectric element, no consideration is given to a piezoelectric ceramic composition itself constituting a piezoelectric portion. Therefore, there has been a problem that sufficient piezoelectric properties cannot always be obtained like the aforementioned piezoelectric element, that an increase of the flexural displacement obtained is very small relative to an increase of voltage in a region of a high electric field, and further that it has insufficient durability.
The present invention has been completed in view of the aforementioned problems and aims at providing a piezoelectric element which has very high piezoelectric properties, which is excellent in vibration transmittability between a substrate and a piezoelectric portion, where linearity of a flexural displacement to voltage is ensured till a high electric region, and which has high durability even in use with a large flexural displacement for a long period of time and a method for production thereof.
The present inventor made a study in order to solve the aforementioned problems. As a result, the present inventor found out that when there is used a piezoelectric material made of a piezoelectric ceramic composition having a specific composition of a PbMg1/3Nb2/3O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition with a part of Mg being substituted by Ni, the densified piezoelectric portion having an even polarization structure can be obtained even when a piezoelectric material is superposed on a substrate and then heat-treated, whereby the aforementioned problems can be solved. The present invention has been completed based on the above finding.
That is, according to the present invention, there is provided a piezoelectric clement comprising;
a ceramic substrate,
a piezoelectric portion made of a piezoelectric ceramic composition containing a Pb(Mg, Ni)1/3Nb2/3O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition being represented by the following general formula (1) as a main component, and
an electrode;
wherein said electrode is electrically connected to said piezoelectric portion, and said piezoelectric portion is solidly attached to the ceramic substrate directly or via said electrode:
Pbx{(Mg1-yNiy)1/3xc3x97aNb2/3}bTicZrdO3xe2x80x83xe2x80x83(1) 
wherein 0.95xe2x89xa6xxe2x89xa61.05; 0.05xe2x89xa6yxe2x89xa60.20; 0.90xe2x89xa6axe2x89xa61.10; b, c and d are decimals falling in a range surrounded by (b,c,d)=(0.550, 0.425, 0.025), (0.550, 0.325, 0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475, 0.425) and (0.375, 0.425, 0.200) in the coordinates with coordinate axes of said b, c and d, and b+c+d=1.000.
According to the present invention, there is further provided a piezoelectric element comprising:
a ceramic substrate,
a plurality of piezoelectric portion made of a piezoelectric ceramic composition containing a Pb(Mg, Ni)1/3Nb2/3O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition represented by the above general formula (1) as a main component, and
a plurality of electrodes;
wherein said plurality of piezoelectric portions are laminated with interposing negative electrodes and positive electrodes of said electrodes alternately in each gap between said piezoelectric portions and a lowermost piezoelectric portion is solidly attached to the ceramic substrate directly or via said electrode.
In any of the piezoelectric elements of the present invention, it is preferable that the ternary system solid solution composition has an average particle diameter of 1-10 xcexcm with a maximum particle diameter being 5 times as large as the average particle diameter or less.
According to the present invention, there is further provided a piezoelectric element comprising:
a ceramic substrate,
a piezoelectric portion made of a piezoelectric ceramic composition containing a Pb(Mg, Ni)1/3Nb2/3 O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition represented by the aforementioned general formula (1) as a main component, and said piezoelectric ceramic composition contain particles having NiO as a main component on the surface and/or in the interior thereof, and
an electrode;
wherein said electrode is electrically connected to said piezoelectric portion, and said piezoelectric portion is solidly attached to the ceramic substrate directly or via said electrode.
Also, in a piezoelectric element where this NiO particle is present, it is preferable that a ternary system solid solution composition has an average particle diameter or of 1-10 xcexcm with the maximum particle diameter being 5 times as large as the average particle diameter or less. In addition, a particle having NiO as the main component may include only NiO or a solid solution of NiO and MgO.
In any of piezoelectric elements of the present invention, it is preferable that Ni is dispersed in the piezoelectric ceramic composition in such a concentration gradient that the concentration of Ni becomes higher from the interface between the piezoelectric portion and the substrate towards the thickness direction of the piezoelectric portion.
Pb in the piezoelectric ceramic composition may be replaced by at least one kind of element selected from the group consisting of Sr, Ca and Ba, by 2 to 10 mole %. Pb in the piezoelectric ceramic composition may also be replaced by La by 0.2 to 1.0 mole %.
It is preferable that the ceramic substrate has a thickness of 3 xcexcm to 1 mm, that the piezoelectric portion has a thickness of 1 to 300 xcexcm, that the ratio of the thickness of the substrate to the thickness of the piezoelectric portion (the thickness of the ceramic substrate/the thickness of the piezoelectric portion) is 0.1 to 30, and that a cross-section of the ceramic substrate in the thickness direction has a W-like shape having three inflection points.
According to the present invention, there is further provided a method for producing a piezoelectric element comprising the steps of:
superposing a piezoelectric material containing a Pb(Mg, Ni)1/3Nb2/3O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition represented by the following general formula (1) as a main component and on a ceramic substrate or on an electrode formed on the ceramic substrate, and
subjecting the superposed piezoelectric material to a thermal treatment in an atmosphere where 0.03-0.5 mg/cm3 (NiO conversion amount per unit volume of a space in a container) of a atmosphere-controlling material having the same composition as the piezoelectric material is coexisted:
Pbx{(Mg1-yNiy)1/3 Nb2/3}bTicZrdO3xe2x80x83xe2x80x83(1) 
wherein 0.95xe2x89xa6xxe2x89xa61.05; 0.05xe2x89xa6y0.20; 0.90xe2x89xa6axe2x89xa61.10; b, c and d are decimals falling in a range surrounded by (b,c,d)=(0.550, 0.425, 0.025), (0.550, 0.325, 0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475, 0.425) and (0.375, 0.425, 0.200), in the coordinates with coordinate axes of said b, c and d, and b+c+d=1.000.
According to the present invention, there is further provided a method for producing a piezoelectric element comprising the steps of:
superposing a piezoelectric material containing a Pb(Mg, Ni)1/3Nb2/3O3xe2x80x94PbZrO3xe2x80x94PbTiO3 ternary system solid solution composition represented by the aforementioned general formula (1) as a main component on a ceramic substrate or on an electrode formed on the ceramic substrate, and
subjecting the superposed piezoelectric material to a thermal treatment in an atmosphere;
wherein a previously heat-treated container and a previously heat-treated setter is used as a container for housing said electrode on which the piezoelectric material is superposed and a setter for mounting the piezoelectric material thereon in the presence of 0.03-0.5 mg/cm3 (NiO conversion amount per unit volume of a space in a container) of an atmosphere-controlling material having the same composition as the piezoelectric material.
The piezoelectric element according to the present invention can be used for condensers or various sensors, as a dense and small dielectric element or pyroelectric element.